The advent of atmospheric pressure ionization (API) has resulted in an explosion in the use of LC/MS analysis. There are currently three main API techniques: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). Each of these techniques ionizes molecules through a different mechanism, and none of the mechanisms are capable of ionizing the entire range of molecular weights and compositions that may be included in a widely varied sample.
Multiple mode ionization sources (“multimode sources”) have been developed which address this difficulty by employing ESI in combination with either APCI or APPI in a single device, so that analytes that are not ionized by the ESI source may be ionized by the secondary ionization mechanism.
Example embodiments of multimode ionization sources are described in U.S. patent application Ser. No. 10/640,176 and its parent application Ser. No. 10/245,987, mentioned above. In brief, in these devices, ions and vapor generated by the ESI source (“ESI ions”) are entrained by a gas and guided toward the vacuum entrance by a combination of gas dynamics and electric fields. Along the trajectory to the vacuum entrance, the ions and vapor enter a volume in which the secondary APCI or APPI source is operative. It has been found that in practice, both types of secondary sources can have a deleterious effect upon ESI ions as they move toward the vacuum entrance. In the case of APCI, it has been found that the corona current emanating from the corona needle can interfere with the movement of the ESI ions toward the vacuum entrance. While the use of a counter electrode to control the corona current can be helpful, the corona current can still be difficult to control. When APPI sources are used, in addition to photoionizing neutral analyte molecules, photons interact with the previously-created ESI ions, which can have a degrading effect upon ESI signals.
It would therefore be advantageous to provide an ionization source that protects a substantial number of ESI ions from the APCI and APPI processes and thereby ensures the quality of the detected ESI signal.